/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution
{
public:
    ListNode *deleteDuplicates(ListNode *head)
    {
        if (head == nullptr)
        {
            return nullptr;
        }
        ListNode *node = head;
        while (node->next != nullptr)
        {
            if (node->next->val == node->val)
            {
                ListNode *next = node->next->next;
                delete node->next;
                node->next = next;
            }
            else
            {
                node = node->next;
            }
        }
        return head;
    }
};